A novel zinc(II) complex with the ligand 2,2′,2′′-(1,4,7-triazanonane-1,4,7-triyl)triacetate (NOTA)

The zinc(II) complex with NOTA [2,2′,2′′-(1,4,7-triazanonane-1,4,7-triyl)triacetate] has previously been synthesized and studied in solution, but was not isolated. The corresponding title ZnII complex pentasodium(I) bis{[2,2′,2′′-(1,4,7-triazanonane-1,4,7-triyl)triacetato]zinc(II)} tris(perchlorate) methanol solvate, Na5[Zn(C12H18N3O6)]2(ClO4)3·CH3OH, was crystallized as a sodium perchlorate double salt in methanol solution. The asymmetric unit contains two independent [Zn(NOTA)]− complex anion entities, five sodium cations, three perchlorate anions and a methanol solvent molecule. The two ZnII cations exhibit a distorted trigonal-prismatic N3O3 coordination with a bifacial arrangement of the donor atoms. Neither the methanol solvent molecule nor the perchlorate anions appear to be coordinated to the Zn centres. The crystal structure shows a layer arrangement parallel to (001) generated by interactions between the [Zn(NOTA)]− units, the Na+ cations, two ClO4 − units and the methanole molecule, leading to an overall layer composition of [Na5[Zn(C12H18N3O6)]2(ClO4)2 .CH3OH]+. The third ClO4 anion is isolated and situated between the layers without any significant interactions.


Comment
Our starting objective was the synthesis of NOTA (1,4,N', complexes in aqueous solutions and its isolation as salts of the type [X][M(NOTA)], where X + is a monovalent cation and M is Zn II or Cd(II). We also developed a species distribution diagram for NOTA complexes in aqueous solution based on the experimental data obtained by Geraldes et al. (1985). From these experiments we concluded that the anionic salts [Zn(NOTA)]and [Cd(NOTA)] -, due to their extreme solubility in aqueous solution, do not precipitate as neither sodium nor the alkylammonium salts. These difficulties for isolating the complexes led us to synthesize the Zn II and Cd(II) NOTA complexes in methanol, a solvent in which the sodium salts are less soluble than in aqueous solution. A Zn II complex with a 1:1 composition was prepared by reaction of the NOTA ligand L with hydrous Zn II perchlorate in a 1:1 molar ratio of metal:ligand. This complex was synthesized by a single-step procedure as described and the reaction revealed a pure product that was also characterized by ESI-MS and 1 H-NMR spectroscopy.
The molecular structure of the complex entity [Zn(NOTA)]and selected bond lengths (Å) and angles (°) of the coordination environment of Zn II are given in Fig. 1 and Table 1, respectively. The asymmetric unit contains two independent mononuclear complex [Zn(NOTA)]entities, five sodium cations, three perchlorate anions and a methanol solvent molecule. The coordination environment, distances and angles of both independent [Zn(NOTA)]molecules are similar. When the metal centre coordination requirements do not favour an octahedral environment, the metal core geometry in NOTA complexes is trigonal-prismatic (Wieghardt et al., 1982). Thus, the Zn II centres present a six-coordinated N 3 O 3 core in a distorted trigonal-prismatic arrangement. Each Zn atom is bound to three N atoms from the macrocyclic backbone and three O atoms supplementary materials sup-2 Experimental Synthesis of the macrocycle NOTAH 3 : The ligand NOTA was prepared from its triazamacrocycle precursor TACN by alkylation with bromoacetic acid using a modification of a previously reported method (Desreux, 1980). TACN and Na-OH were dissolved in water, and to the solution was added a bromoacetic acid/ NaOH aqueous solution at 273 K. The reaction mixture temperature was raised to 323 K, and a NaOH aqueous solution was added. The mixture was maintained at 323 K under stirring for 5 d. Then, concentrated hydrobromic acid was added until a pH of ~ 7 was reached. NO-TAH 3 does not precipitate from aqueous solutions in a well-defined state, thus a purification stage was needed. After a liquid-liquid extraction with n-butanol, a white powder characterized as a salt of the expected ligand was finally obtained. The correspondent Cd(II) complex was also obtained in methanolic solution, but it has not been isolated in crystalline form.

Refinement
The absolute structure parameter was refined (Flack, 1983) and points to racemic twinning, with a ratio of the twin fractions of approximately 3:2. The hydrogen atoms attached to the carbon atoms were located in their calculated positions and refined using a riding model with U(H) equal to 1.2× U eq (1.5 for methyl groups) of the parent atom and C-H= 0.97 Å. The hydrogen atom attached to the oxygen atom in the methanol molecule was localized in a Fourier map and refined with U iso constrained to be 1.5 × U eq of the O atom.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , convention-